made the leftdock follow the presentation arrows

src/qml/presenter/LeftDock.qml

@@ -8,6 +8,7 @@ import QtMultimedia 5.15
 /* import QtAudioEngine 1.15 */
 import org.kde.kirigami 2.13 as Kirigami
 import "./" as Presenter
+import org.presenter 1.0
 
 ColumnLayout {
     id: root
@@ -292,4 +293,8 @@ ColumnLayout {
         serviceItemModel.addItem(name, type, background,
                                  backgroundType, lyrics);
     }
+
+    function changeItem() {
+        serviceItemList.currentIndex = currentServiceItem;
+    }
 }

src/qml/presenter/Presentation.qml

@@ -168,6 +168,7 @@ Item {
         if (changed) {
             currentServiceItem++;
             loadVideo();
+            leftDock.changeItem();
         }
     }
 
@@ -187,6 +188,7 @@ Item {
         if (changed) {
             currentServiceItem--;
             loadVideo();
+            leftDock.changeItem();
         }
     }